Question

I want to use parser actions with basic file io (Java), e. g. PrintWriter in an ANTLR grammar. Must I use the superClass option or can I use @header? In both cases how can I declare the PrintWriter-object and how must I handle the exceptions?

Answer 1

The option superClass=... is used to let your Parser extend a custom class. So, I don’t think that is what you’re after.

Everything inside the @header section will be placed at the start of your Parser class. This is used to import classes:

@header {
  import java.io.PrintWriter;
}

Note that @header {...} is short for @parser::header {...}. You can also define: @lexer::header {...} for your lexer.

And inside @member {...} (or: @parser::member {...}, @lexer::member {...}) sections, you can add instance variables and methods that can be used inside either the Parser or Lexer:

@header {
  import java.io.PrintWriter;
}

@members {
  PrintWriter writer;
}

A small demo of a grammar whose parser will write the parsed numbers to a specific writer:

grammar T;

@header {
  import java.io.PrintWriter;
}

@members {
  PrintWriter writer;

  public TParser(TokenStream input, String fileName) {
    super(input);
    try {
      writer = new PrintWriter(fileName);
    } catch(Exception e) {
      e.printStackTrace();
    }
  }
}

parse
  :  numbers EOF
  ;

numbers
  :  (NUM {
            writer.println("parsed: " + $NUM.text);
            writer.flush();
          }
     )+
  ;

NUM : '0'..'9'+;
WS  : ' ' {skip();};

which can be tested with:

import java.io.File;
import org.antlr.runtime.*;

public class Main {
  public static void main(String[] args) throws Exception {
    String source = "42 500000000 666";
    TLexer lexer = new TLexer(new ANTLRStringStream(source));
    TParser parser = new TParser(new CommonTokenStream(lexer), "log.txt");
    parser.parse();
  }
}

If you run the class above, a file called log.txt has been created containing:

parsed: 42
parsed: 500000000
parsed: 666

Note that there is a strict order of all these @... and options {...} etc. instances:

1. grammar definition
2. options block (no @ sign!)
3. tokens block (no @ sign!)
4. @header block
5. @members block

grammar T;

options {
  // options here
}

tokens {
  // imaginary tokens here
}

@header  { 
  // ... 
}

@members { 
  // ... 
}

---

EDIT

ANTLRStarter wrote:

How can I add code which is executed at the end of the the lexer/parser class?

There’s no built-in functionality for such a thing. But you can easily create a custom method wrapUp() in your parser:

@members {

  // ...

  private void wrapUp() {
    // wrap up your parser in here
  }
}

and then automatically call that method from the entry point of your grammar like this:

parse
@after {this.wrapUp();}
  :  numbers EOF
  ;

Any code placed in the @after {...} block of a rule is executed when everything in the rule is properly matched.